This invention relates to a device for discontinuing and automatically restoring the operational function of a spring brake actuator, comprising a cylinder, a piston, a spring actuating the piston in a working direction for brake application at falling fluid pressure at the opposite side of the piston, a non-rotatable piston rod, and in the force transmitting path between the piston and the piston rod two means coaxial with the piston rod and in non-self-locking thread engagement with each other, of which means one is rotatably journalled and the other at least at the spring force transmission is non-rotatable.
Devices for discontinuing and automatically restoring the operational function of a spring brake actuator are needed for the case that no fluid pressure is available for compressing the spring and thus for releasing the brakes and that the vehicle on which the actuator is mounted must be moved. Thus the force transmitting path in the actuator from the spring to the piston rod may be broken from the outside but must be restored automatically at the return of fluid pressure for compressing the spring.
A device of the kind referred to above is shown in Swedish Pat. Specification No. 7314472-7 U.S. Ser. No. 639,407 filed Dec. 10, 1975 now U.S. Pat. No. 3,994,206 which is a continuation of U.S. Ser. No. 514,929 filed Oct. 15, 1974, now abandoned. This device comprises a rotatable nut in non-self-locking engagement with a threaded spindle or piston rod. There is a releasable one-way clutch between the piston and the nut. Two embodiments are shown, both with certain drawbacks. In the first embodiment there is a control means for the clutch following the piston movements and extending out of the actuator. This solution gives rise to sealing problems and may even in certain cases be less reliable. In the second embodiment the whole spring force is transmitted via the clutch, which means that a great manual force is needed for releasing the clutch.